Potentiometer brush



Aug. 14, 1956 c. A. MOUNTEER 2,759,081

POTENTIOMETER BRUSH Filed March 15, 1954 6221 4.15 A. Mam/755e,

IN VEN TOR.

1 flrrazwsys.

QAifiL- Unite States 2,759,081 Patented Aug. 14, 1956 POTENTIOMETER BRUSH Carlyle A. Mounteer, Pasadena, Calif, assignor to G. M. Giannini & Co. Inc., Pasadena, Calif a corporation of New York Application March 15, 1954, Serial No. 416,019

4 Claims. (Cl. 201-62) This invention has to do with wire-wound potentiometers and, more particularly, with an improved type of brush for movably engaging the coil surface of such Potentiometers.

The outer surface of a wire-wound potentiometer coil is necessarily a somewhat uneven surface. That unevenness is due in part to the circular cross section of the wire with which the coil is Wound, and in part to irregularities of dimensions from one turn of the coil to another. The diameter of the wire ordinarily is not strictly uniform, and the spacing between adjacent turns, even if it is determined primarily by an insulative coating on the wire, is somewhat variable.

In a potentiometer intended for applications requiring the highest precision, it is difiicult to provide a brush that has sufiicient flexibility to accommodate itself to the irregularities of the coil surface, and that is at the same time sufiiciently rigid to provide accurately reproducible definition of the eifective position of the brush along the coil. It is also difficult to provide a brush with a contact face that is wide enough in the direction of its motion to insure smooth sliding movement over the irregularities of the coil surface and effective electrical contact at all times, and that is at the same time narrow enough to limit the electrical contact to a small and accurately defined portion of the coil surface. Moreover, with brushes of previously available form, progressive wear of the contact face of the brush tended to change its form, so that a brush initially having optimum form would retain that form only for a limited period of use.

While those difiiculties may be present in the operation of any potentiometer in which high precision is required, they are especially severe in precision potentiometers that are wound with extremely fine wire. Particularly in aircraft instrumentation, the use of small highly sensitive potentiometers is becoming increasingly important for translating relatively small movements of sensing elements, such, for example, as pressure responsive capsules, into corresponding electrical voltages for transmission to an indicating or recording instrument. Potentiometers for such use must be wound with extremely fine wire, both to increase the number of turns per unit of coil length for increased sensitivity and to increase the number of ohms per turn. In potentiometers for service of that type it is not uncommon to employ wire having a diameter of one thousandth of an inch or less. The present invention is particularly concerned with problems of the type described that arise from, or are aggravated by, the use of very fine wire in the potentiometer coil.

The present invention provides a particularly effective solution to those problems. It further provides a potentiometer brush that is economical and convenient to produce, and that functions reliably and with uniformly high precision independently of wear of the contact surface.

More particularly, the invention provides a potentiometer brush that is suificiently flexible to move smoothly over irregularities of the coil surface, while at the same time being sufficiently rigid to avoid chattering and to define the contact position with satisfactory accuracy. The area of contact between the brush and coil is sulficiently narrow in the direction of brush movement to produce a sharply and reproducibly defined voltage, and at the same time is sutficiently wide to insure eifective electrical contact and smooth transition from one turn of the coil to the next.

Fig. 1 is an elevation, partly schematic, representing an illustrative potentiometer in accordance with the invention at typical actual scale;

Fig. 2 is a fragmentary elevation corresponding to a portion of Fig. 1 at enlarged scale;

Fig. 3 is a plan corresponding to Fig. 2;

Fig. 4- is a fragmentary section, partly schematic and at greatly enlarged scale, illustrating the invention;

Fig. 5 is a fragmentary section corresponding to Fig. 4, but representing a typical previous form of potentiometer brush;

Fig. 6 corresponds to Fig. 5, and illustrates the effect of wear; and

Fig. 7 is a fragmentary perspective, representing another illustrative embodiment of the invention.

Referring to the drawings, a wire-wound potentiometer coil is represented at 10, with a core 12 on which the wire 14 is wound. The core may be supported by any suitable means, such as the brackets is and 1'7. Core 12 may be of circular section, or may have any of the many forms well known in the art. A brush support is represented in Fig. 1 at 26, mounted in any suitable manner for movement longitudinally of the potentiometer coil. Such mounting for support 2t]? is represented schematically by the rod 22, which is fixedly mounted on brackets 16 and 17 parallel to the coil, and upon which the brush support may be considered to be slidable in response to mechanical movement of a sensing device such as a pressure responsive bellows, not shown. Many types of mechanical or other linkage are known in the art, suitable for connection between such a sensing element and a potentiometer brush, and need not be described in detail here.

A potentiometer brush is indicated at 36}, mounted on support 29 for movement therewith, and making sliding contact with the coil surface. The track of the contact face of the brush as it slides along the coil surface will be referred to as the contact axis of the coil, and is ordinarily parallel to the geometrical axis of the coil. However, the cross section of a potentiometer coil is not necessarily uniform, but may, for example, vary progressively over its length in order to produce a predetermined type of non-linear response. The coil axis then may not be well-defined, and the contact axis of the coil provides a more useful reference direction. The brush, as shown illustratively in Fig. l (and in Figs. 2 and 3, to be described) comprises a relatively long spring portion with a contact portion at its end.

Brushes of that general type are well known in the art, a typical contact formation being shown for the sake of comparison in Figs. 5 and 6. That structure includes a relatively long arm 32 of fiat spring material with its outer end bent to form a convexly curved contact surface, indicated at 34. An advantage of such a curved contact face is the ease with which such a surface can slide over the irregularities in the coil surface. However, such a curvature of the brush surface has the serious disadvantage that it is not permanent, but tends to wear relatively rapidly with use, producing a flat area, as indicated at 36 in Fig. 6, which contacts a progressively larger number of coil turns as the wear progresses. If the spring pressure was originally suitable for producing reliable electrical contact between the brush face and the 3 few turns of the coil that the initial curvature at 34 engaged, then the same pressure may be inadequate for reliable action when the flattened brush face 36 must contact a relatively large number of coil turns. Furthermore, the longitudinal position on the coil of the extended contact face 36 is not sharply defined.

That problem is especially important in precision potentiometers that are wound with extremely fine wire, since an amount of brush wear that would not appreciably alter brush engagement with larger wires may produce serious changes in the manner of engaging very small wires. For example, if the wire diameter in Figs. 5 and 6 is one mil, the total brush wear represented in Fig. 6 is less than one mil. Yet that relatively small amount of wear has fundamentally changed the nature of the contact. The possibility of completely avoiding that problem is therefore of crucial importance in precision potentiometers of the type having wire diameter of the order of one mil or less.

It has been discovered that smooth and effective movement of the brush over the coil surface does not require a curved contact surface of the type illustrated in Fig. 5. It is feasible, instead, even in precision potentiometers of the type described, to allow the end of the brush to abut the coil surface directly, for example in the manner shown in Figs. 1 to 4. As is seen clearly from the drawings, that structural arrangement has the great advantage that even relatively great wear of the brush does not alter at all the dimension of its contact face measured longitudinally of the coil, and hence does not significantly affect the nature of the contact. However, satisfactory operation with that type of engagement has been found to require certain rather definite dimensional conditions upon the structure of the brush. If those conditions are not met, the brush may chatter as it passes over successive turns of the coil, and may even catch on irregularities of the coil surface; or the position of the brush may not be sufiiciently sharply defined to give the required precision of response.

A preferred type of potentiometer brush in accordance with the invention is shown schematically and at greatly enlarged scale in Fig. 4, and includes a substantially straight contact arm 40 extending perpendicularly to the coil surface and having one end face 42 directly abutting that surface. The other end 43 of contact arm 49 is mounted on an arm support, which is schematically indicated in Fig. 4 as the relatively heavy block 47. Any suitable type of effectively rigid connection between contact arm 40 and support 47 may be employed, such, for

example, as soldering or welding; or the arm and support may be of unitary construction. An important feature of the invention is the discovery that a definite dimensional relationship between contact arm 40 and the diameter of the wire 14 of the potentiometer coil leads to smooth and reliable operation of the described brush. It has been found that the length of arm 49 should be between about fifteen and about twenty-five times the wire diameter; and that the thickness of the contact arm in the direction of its movement should be substantially uniform over the length of the arm and approximately equal to three times the wire diameter. The transverse arm dimension in the direction normal to the paper as shown in Fig. 4 is not particularly critical, but is preferably large compared to the two dimensions already defined.

Potentiometer coils of the type described are ordinarily wound with adjacent turns relatively closely spaced. Such turns may, for example, be separated only by the very thin coating of insulating material that is applied to the wire before it is wound. The pitch of the winding is then substantially equal to the diameter of the wire. In the above definitions of brush dimensions, it is to be understood that the coil pitch may ordinarily be substituted for the wire diameter without appreciably affecting the result. In those few instances in which the pitch diameter is appreciably greater than the wire diameter, the former is preferred as a reference dimension.

With the described relationship between the dimensions of contact arm 46 and the coil wire, it has been found that, in spite of the perpendicular position of arm 40, the latter does not chatter or jump in sliding over the coil surface, as might be expected. Apparently the relation between the flexibility of the arm and the length of the lever arm at which the frictional force of sliding contact is applied is correct to produce smooth and relatively wear-free movement, and hence to maintain definite and accurately defined electrical contact at all times.

Contact arm 40 need not be mounted directly on a relatively massive support, as indicated in Fig. 4. A preferred manner of supporting the contact arm is shown illustratively in Figs. 2 and 3. A relatively long support arm 50 is provided, extending parallel to the direction of brush movement. Arm 50 has one end rigidly mounted on a support 56, Which may correspond to support 20 of Fig. 1, and its other end carries the contact arm 54. The arms 54 and 50 may be rigidly joined, as by soldering, but are preferably fabricated from a single piece of resilient sheet metal, bent through substantially a right angle at 52. It has been found that the brush moves more smoothly over the coil surface, and at the same time is less subject to inaccuracy of longitudinal position, if the bend at 52 is not absolutely sharp but is a smooth bend having a relatively large radius of curvature. The preferred value for the radius of curvature of bend 52 is approximately one-half of the effective length of contact arm 54, that effective length being taken as the distance from the contact end 55 of the arm to the plane of support arm 50. The contact arm is then substantially straight over a major portion of its length, typically onehalf its length, the remainder of its length being curved as indicated at 52.

The length of support arm 50 is preferably at least ten times that of contact arm 54, so that yielding movement of arm 50 in response to irregularities of the coil surface will produce substantially rectilinear movement of contact arm 54 normal to the coil surface, with only negligible rotation of the contact arm. The contact axis of the coil, already defined, is indicated in Fig. 3 at 19, support arm 50 extending substantially parallel to that axis and in parallel spaced relation to the coil surface.

Another illustrative manner of supporting a contact arm in accordance with the invention is illustrated in Fig. 7, also at greatly enlarged scale. The contact arm is there indicated at 64, and may be formed as shown by bending at right angles a portion of an elongated piece of resilient sheet metal, the main body of the sheet forming a resilient support arm as in the modification of Figs. 1 to 3. However, in the modification of Fig. 7 the support arm extends transversely of the direction of movement of the brush. As shown in Fig. 7, one end of support arm is rigidly mounted on the support member 66, which is movable parallel to the contact axis of the coil, indicated at 1%. Member 66 may, for example, be slidable along the fixed rod 68. Support arm 60 preferably comprises two oblique legs 61 lying in a common plane and typically of integral construction. That structure combines great rigidity in the direction of brush movement with desirable flexibility in the direction per- 7 pendicular to the coil surface.

The dimensional relations already defined apply to contact arm 64 of Fig. 7. That is to say, the thickness of arm 64 in the direction of brush movement (parallel to the rod 68 in the present instance) is approximately three times the pitch of the coil winding; the length of the contact arm, measured perpendicularly to the coil surface, is between about fifteen and about twenty-five times the coil pitch; and the dimension in the plane of the coil surface and transverse of the direction of movement is preferably considerably greater than the length of the greener arm. Furthermore, if contact arm 64 is formed by bending a portion of support arm 60 out of its plane, as indicated at 62, the radius of curvature of that bend is preferably approximately equal to one-half the effective length of the contact arm that is thereby produced.

In the embodiment of Figs. 2 and 3 and also in that of Fig. 7, the contact arm may alternatively be fabricated separately from the support arm, and may be fixedly mounted on the outer end of the support arm in any suitable manner, such as by soldering or welding. The contact arm may then be straight throughout the whole of its length, as indicated at 40 in Fig. 4, support 47 of that figure then representing the end of support arm 50 or 60. That non-integral structure offers the advantage that each of the arms may be of different material, specially selected for its primary function. However, the described unitary structure, with the radius of curvature of the bend (52 or 62) meeting the condition already defined, has been found to give particularly smooth and well-defined movement.

Electrical connection may be made in any suitable manner to the described potentiometer brushes, for example by means of a flexible wire electrically connected, as by soldering, to the support arm. Such a wire is indicated at 63 in Fig. 7.

It will be understood that a contact arm in accordance with the invention may be supported in many other particular manners, and that many other changes may be made in the particulars of the described embodiments without departing from the proper scope of the invention, which is defined by the appended claims.

I claim:

1. In combination with a wire-wound potentiometer coil having a predetermined and substantially uniform coil pitch, a potentiometer brush comprising a support portion mounted for movement longitudinally of the coil and an elongated arm portion one end of which is rigidly mounted on the support portion, the other end of the arm portion directly abutting the surface of the coil and thereby forming the only electrical contact between the brush and the coil, the length of the arm portion being between about fifteen and about twenty-five times the coil pitch, the arm portion having a cross-sectional area that is elongated transversely of the brush movement, the dimension of that area parallel to the brush movement being substantially uniform throughout the length of the arm portion and being approximately three times the coil pitch, the arm being substantially straight and perpendicular to the coil surface throughout at least about that half of its length adjacent its said other end.

2. In combination with a wire-wound potentiometer coil having a predetermined and substantially uniform coil pitch, a potentiometer brush comprising a support movable longitudinally of the coil, a resilient support arm having one end rigidly mounted on the support and extending in parallel spaced relation to the coil surface, a contact arm having one end rigidly mounted on the other end of the support arm and extending substantially normally to the coil surface with its other end directly abutting that surface and thereby forming the only electrical contact between tthe brush and the coil, the length of the contact arm being between about fifteen and about twenty-five times the coil pitch, and the contact arm having a cross-sectional area that is elongated transversely of the brush movement, the dimension of that area parallel to the brush movement being approximately three times the coil pitch.

3. In combinatiton with a wire-wound potentiometer coil having a predetermined and substantially uniform coil pitch, a potentiometer brush comprising a support mounted for movement longitudinally of the coil, a single strip of resilient sheet metal having one end rigidly mounted on the support, the main portion of the strip extending in parallel spaced relation to the coil surface, a relatively small portion of the strip at its outer end being bent through an arc of substantially degrees to form a contact arm, the contact arm lying substantially in a plane perpendicular to the direction of brush movement and to the coil surface and having its free end directly abutting that surface and thereby forming the only electrical contact between the brush and the coil, the length of said contact arm being between about fifteen and about twenty-five times the coil pitch, the transverse dimension of the contact arm measured parallel to its movement being approximately three times the coil pitch, and the radius of curvature of the are between the contact arm and the main portion of the strip being approximately equal to one half the length of the contact arm.

4. In combination with a potentiometer coil wound with wire of a diameter approximately equal to 0.001 inch, adjacent turns of the coil being closely spaced, a potentiometer brush comprising a support mounted for movement longitudinally of the coil, a single contact arm of generally flat section lying substantially in a plane perpendicular to the direction of brush movement and having one end rigidly mounted on the support, the other end of the arm directly abutting the surface of the coil and thereby forming the only electrical contact between the brush and the coil, the length of the arm being between about 0.015 and about 0.025 inch and the transverse dimension of the arm measured parallel to its movement being approximately 0.003 inch.

References Cited in the file of this patent UNITED STATES PATENTS 1,460,239 Gehrig June 26, 1923 2,611,847 Scott Sept. 23, 1952 FOREIGN PATENTS 321,888 Great Britain Nov. 21, 1929 

